1. Field of the Invention
This invention relates to a method and apparatus of cementing a casing in a wellbore in which the casing will subsequently be heated to a higher temperature, causing it to want to elongate. In particular, it concerns a method whereby the cement is circulated using conventional methods, the casing is stressed and the cement is allowed to set.
2. Setting of the Invention
In the search for oil and gas, boreholes are drilled deep into the earth. These holes are lined with casing, which is usually heavy steel pipe, and cement is forced into the annulus between the casing and the borehole wall. In most cases, the temperature of the fluid if flowing through the casing is such that the temperature of the casing does not vary to any degree from that at which it was set. However, in a growing number of situations, the fluid flowing through the wellbore is of such high temperatures that the casing is heated to a much higher temperature then when the casing was set. This is true of thermal wells. A thermal well can be a well in which steam or other hot fluids are injected down through a tubing string suspended in the wellbore to aid in the recovery of the fluid from the underground formation, or which the produced fluid from a formation are at a very high temperature. The increased temperature in a thermal well causes the casing to try to elongate. It has been found that if the casing is hung and cemented and large temperature differences are added to the casing, the tensile stress reduction for the fixed cmemented casing is approximately 200 psi per degree F change.
In conventionally cementing a casing string in a wellbore, the casing string is reciprocated and rotate during the placing or circulation of the cement between the outer wall of the casing and the wellbore.
3. Relevant Publications
U.S. Pat. No. 3,976,139, Lawrence B. Wilder, issued Aug. 24, 1976 and entitled "Anchoring for Tensing Casing and Thermal Wells" concerns a method for pretensioning a casing string and cementing it in place while this tension condition is maintained. The method of Wilder requires that the casing string be anchored to the formation wellbore wall after the cement has been circulated. Such an arrangement does not permit the casing to be placed under stress and rotated in the same operation. Further, the method does not allow the casing string to be put under stress during the circulation of the cement. Still further, the method does not allow the casing string to be rotated while being maintained under stress.
The instant invention described herein has a different anchoring system and permits cement to be placed in the casing annulus in the conventional manner while stressing is placed upon the casing string.